I didn't notice in your post, but this looks like a 5 gallon recipe judging from the honey.
There seems to be several things you may wish to consider before making adjustments. If you want to keep the mead as natural as possible, you might want to balance acidity with honey rather than adding chemicals to balance taste. If you intend to age it a good while, a lower pH is beneficial against bacterial infections. A low pH also makes SO2 additions a bit more effective at the same dosages. "Usually" adding highly acidic fruits to a must brings the pH down.
The simplest thing to do is to add just enough honey to bring the acidity to an acceptable level and then wait a bit. After a bit of aging taste again and if needed add a bit more honey. With time, a bit of the perceived acidity will diminish and the honey will become more apparent.
Low pH, I would consider anything under 3 to be low, means you will notice the acidity on your tongue more since the must's ability to buffer it is low. Increasing the ability to buffer, raising pH, will mean you notice less acidity on your tongue. Changing the pH with buffering compounds means less perceptable acidity.
I guess what I mean with the above crap, I would add honey to balance rather than mess with buffering. My personal preference for these types of berry meads is a slightly tart finish anyway. Raspberry should be a bit tart to me to give the impression of the true fruit character. Making it sweet enough to mask the acidity would diminish the true character of the fruit and make it seem more artificial I guess.
Just the last two cents I have... I'm sure someone will be able to add more helpful info than me on this subject.
